With developments of society and scientific technology, the electronic products become smaller and smaller, resulting that printed circuit boards for connections between different devices and substrates for IC packaging are required to become lighter, thinner, shorter and smaller, while maintaining good electrical properties and thermal properties. In order to meet the above requirements, both conductive wires having smaller size and highly reliable conductive vias having smaller size are two technical requirements that must be satisfied.
There are three typical methods for forming wires: (1) the subtractive method, i.e., forming an etch resistant pattern on the surface of copper foil by film development; removing the exposed copper foil by selectively etching; and then forming a conductive pattern by removing the etch resistant pattern. Referring to FIG. 1 (11—a dielectric layer, 12—a copper layer, 13—a thin film), this process comprises the following steps: step 1, forming a laminated plate of the cover copper layer 12 on the laminated dielectric layer 11 (FIG. 1a); step 2, applying a thin film 13 to form an etch resistant layer through image-transfer and development (FIG. 1b); step 3, selectively etching to remove exposed copper layer (FIG. 1c); step 4, removing the thin film to obtain the conductive pattern (FIG. 1d). This method has a primary disadvantage that the exposed copper is etched sideward when it is etched downwards during etching. Therefore, the copper layer will be over-etched when the thickness of the copper layer is normal. As a result, it is difficult to form wires, thus limiting application of the subtractive method in forming fine lines.
(2) The fully-additive method, i.e., a method of forming a conductive pattern by selectively electroless-depositing copper after exposing a dielectric substrate comprising a photo-sensitive catalyst according to a wire pattern. Referring to FIG. 2 (21—a photo-sensitive resin, 22—an exposed area, 23—an electroless thick copper). This method comprises the following steps: step 1, selecting a photo-sensitive resin containing a photo-sensitive catalyst (FIG. 2a); step 2, forming a plating pattern by exposing (FIG. 2b); step 3, selectively electroless plating thick copper to form wires (FIG. 2c). This method is suitable for forming a fine line. It, however, requires a specific substrate. Therefore, it has a disadvantage of high cost, and it has not been perfected yet.
(3) The semi-additive method, comprising chemically depositing copper onto a dielectric substrate to form a thin copper foil; pattern plating to thicken a conductor; and then removing the excessive thin copper foil by rapid etching to form a conductive pattern. Referring to FIG. 3 (31—a substrate, 32—an electroless deposited copper layer, 33—a thin film, 34—a plated copper layer), this method comprises the following steps: step 1, electroless plating copper onto a substrate 31 to form a seed layer 32 (FIG. 3a); step 2, providing a thin film 33 and forming a plating resistant layer by image-transfer (FIG. 3b); step 3, pattern plating to thicken wires (FIG. 3c); step 4, removing the film (FIG. 3d); and step 5, rapidly etching to form a conductive pattern (FIG. 3e). Because the copper layer obtained by electroless depositing is very thin, so that it is easy for etching, this method is suitable to form a fine wire.
Additionally, there are other methods for forming lines, for example, a modified semi-additive method relatively widely used, which comprises the following steps: step 1, chemically depositing copper; step 2, panel plating; step 3, forming a plating resistant layer by pattern-transfer; step 4, plating copper and an etch resistant layer according to a pattern; step 5, etching copper to get the conductive pattern after removing the plating resistant layer; step 6, removing the etch resistant layer to obtain the final conductive pattern. This method can obtain finer lines than those obtained by a subtractive method, by controlling the thickness of the copper layer obtained by panel plating. However, when compared with the semi-additive method described above, the ability of the modified semi-additive method to form fine lines is still limited, because the copper layer obtained by plating has a thickness greater than that of the copper layer obtained by electroless plating. Generally, the subtractive method is a traditional process which is applied most widely. But it is limited by its limited ability to form fine lines. Although the fully-additive method is suitable to form fine lines, it requires a specific substrate. Therefore, it has a disadvantage of high cost, and it is still not perfect yet. The semi-additive method is a combination of the fully-additive method and the subtractive method, which is an optimized method for forming fine wires recently.
After forming wires, it is necessary to form interconnections between different layers so that an electrical connection can be formed in multi-layer printed circuit boards. Conductive blind vias or through holes are primarily used to connect different layers. There are many methods for forming conductive blind vias or through holes. Five of them are listed below:
(1) Mechanical perforation, which is the conventional method for forming conductive through holes. For example, the desired through holes are formed in a substrate through a punch or a drill press, and then hollow conductive through holes are formed by depositing or plating copper. Referring to FIG. 4, reference number 41 refers to a substrate, reference number 42 refers to a lower layer of wires, reference number 43 refers to a dielectric layer, reference number 44 refers to a hollow conductive through hole, and reference number 45 refers to an upper layer of wires. The aperture has a greater diameter when formed by the mechanical perforation. Therefore, it is difficult to increase the density of wires.
(2) Optical imaging dielectric or laser perforation technique for forming blind conductive vias. The method for forming blind conductive vias comprises the following steps: forming micro-dents on photo-sensitive dielectric materials by an image-transfer technique, or forming micro-dents directly on dielectric materials by a laser perforation technique; and then forming blind conductive vias by depositing or plating copper, as described in FIG. 5 (wherein, 51—a substrate, 52—a lower layer of wires, 53—a dielectrics layer , 54—an upper layer of wires, and 55—a blind conductive via). The method forms hollow conductive vias. However, the conductivity, thermal conductivity and reliability of the hollow conductive vias are lower than those of solid blind vias. At the same time, this structure cannot provide a stacked via structure, therefore, it is difficult to increase the density of wires.
(3) Japanese patent No. 6-314878 discloses a method of wholely conducting a lower wire layer and forming a resist pattern having an opening portion and depositing metal on a concave section of the resist pattern by means of electrolytic plating to form a pillar conductor. However, the method has a problem that the height of the pillar conductor is easily non-uniform. At the same time, the plating current density cannot be increased, so the formation of the pillar takes a long time.
(4) U.S. Pat. No. 6,555,209B1 discloses a method of forming solid conductor, comprising: coating an etching-resistance metal on the lower wiring layer; forming a plating layer above the said protective metal by electrolytic plating; forming a mask layer on the surface of the plating layer where pillar-like metallic body is formed; etching the said plating layer to form the pillar; etching the protective metallic layer to get the lower layer pattern. However, the method has the next disadvantage. The diameter of the connection pillar with a certain height is smaller than the bottom portion because of the etching factor, which makes it difficult to narrow the gap between the connection pillar needed to form a fine circuit pattern. And so the method is still limited to increase the wire density.
(5) Copper filling plating for forming conductive vias, comprising: forming micro-dents on photo-sensitive dielectric materials by an image-transfer technique, or forming micro-dents, directly on dielectric materials by a laser perforation technique; and then forming blind conductive vias by depositing or plating copper. During the process of plating, not only the copper layer is thickened, but also the via is filled by filling-vias techniques. This method can provide stacked via structure easily and can produce the conductive vias with high reliability. However, in the practical process of plating, the copper layer generally has a greater thickness in order to fill the vias and the wires are formed by subtractive method, which is a disadvantage for the preparation of fine lines.
To summarize the above description, firstly, the semi-additive method is a preferred method to form fine lines; secondly, a pillar-like conductor via is a preferred method to form the electrical connections between different layers, where the stacked via structure can be applied and then the wire density can be increased. However, the conventional semi-additive method cannot provide solid conductive vias, thereby cannot provide stacked vias to connect any layers. On the other hand, there are different kinds of methods as described above which can form solid conductive vias. But the methods still have many disadvantages as described above or are not compatible with the semi-additive process. There is a necessity to provide a method having advantages both of the semi-additive method and solid conductor vias, i.e., easily providing interconnections between any layers by solid conductive vias and a structure of stacked vias, while fine lines can be easily fabricated by a semi-additive method. The present invention provides a technical solution which meets above requirements.